1. Field of the Invention
The invention relates generally to patterned lens structures such as are used in conventional fluorescent lensed troffers, the invention relating particularly to lens structures having performance comparable to conventional lens structures while requiring substantially less material for forming of said lens structures.
2. Description of the Prior Art
Lenses used as covers for fluorescent lighting fixtures and fixtures utilizing other light sources are often referred to in the art as lighting panels, these panels or lenses being primarily used to reduce direct glare from fluorescent lighting fixtures and particularly such fixtures disposed overhead in commercial, office and other environments. In view of the manner in which light is distributed from a light source or sources within such lighting fixtures, these lighting panels or lenses are referred to as "prismatic" even though prisms are not necessarily used in formation of such lenses. Prismatic lenses used in lensed fluorescent troffers or similar lighting fixtures not only act to reduce direct glare by controlling the angle at which light emerges from the lens, these lenses also obscure lamping in the fixture by spreading light concentrations to produce a more aesthetically pleasing appearance.
The functions of prismatic lens structures are well known and are discussed inter alia in U.S. Pat. No. 2,474,317 to McPhail. The "lighting panels" described in this patent include a planar upper face and a lower face covered with "prismatic elements", light rays entering the top of the panel being either refracted downwardly through a lower surface of the panel at useful angles to the vertical, that is, normal to the panel, or are reflected internally by the prismatic elements upwardly through the upper surface of the panel. Formation of the prismatic elements to have straight sides making a proper angle with the normal to the panel causes virtually all light which would otherwise emerge at high angles relative to the normal to the panel to be internally reflected by the prisms or prismatic elements, thereby reducing or eliminating high angle "direct" glare.
While prismatic lenses of widely varying description have previously been devised including lenticular lighting panels such as are described by Harvath in U.S. Pat. No. 5,003,448, a particularly useful prismatic lighting panel is seen to have, on its lower surface, female conical prisms, the apices of which are aligned along 45.degree. diagonals to the edges of the lens and spaced approximately 3/16 inch on center. Intersections of the cones thus form a structure of square cells, the sides of the cells lying along lattice lines running at angles of 45.degree. to the edges of the lens. One example of such a lighting panel or lens is marketed by K-S-H, Inc. of St. Louis, Missouri under the trademarked designation KSH-12, this type of structure being generically known in the art as an A-12 lens.
The ubiquitous usage of lensed troffer lighting fixtures in a wide variety of commercial environments in particular has caused cost pressures to be exerted on the entirety of such fixtures and particularly on the prismatic lens structures forming covers of such fixtures and providing, as aforesaid, light control and reduction of lamp image. Since the plastic or "resinous" material from which these prismatic lenses are formed represents the primary cost of such lenses, these prismatic structures have been formed of increasingly thinner design until the point has been reached whereby even more thin structures are not permitted by geometry in order to further reduce weight. Additional weight reduction steps have involved reshaping of the female conical prisms by rounding straight edges of the conical prisms to make said prisms concave in cross-section. In such reformed prisms, prism apices have often been truncated or rounded off to permit formation of a prismatic lens using less material in its formation. However, while lenses of this type give the general appearance of an A-12 lens, such lenses are less effective optically and are generally known in the industry by another designation such as "pattern-12". Prismatic lenses of this nature provide higher and less sharply defined cut-off angles and therefore are relatively ineffective in controlling direct glare. As prismatic lenses have been made thinner and profiles modified, these lenses have also become less effective in hiding or spreading lamp images when viewed from below. Additionally, changes in prism geometry which have permitted formation of ultralight structures in thicknesses of less than 0.100 inch, typically 0.085 to 0.090 inch, have actually increased weight when formed as thicker lenses. In the manufacture of such lenses, multiple tooling is required such as through the use of a first embossing roll for thicknesses under about 0.090 to 0.100 inch and a second embossing roll for thicker panels in order to maintain a weight as low as possible for a given thickness. The weight of prismatic lenses, particularly of the pattern-12 type, have also been reduced by physically stretching the panel in a lengthwise direction, that is, in the direction of the axes of the fluorescent tubes in a rectangular lighting fixture, after embossing but before complete cooling of the plastic. Such stretching, however, creates stresses in the plastic and distorts the lattice pattern of intersecting prismatic cells.
Prismatic lenses, often referred to as prismatic lighting panels, are described in U.S. Pat. Nos. 2,474,217 to McPhail; 3,988,609 to Lewin; 5,003,448 to Harvath; 5,057,984 to Kelley; 4,542,449 to Whitehead and 5,274,536 to Sato, the disclosures of these patents being incorporated hereinto by reference. As is seen in part from the disclosures of the foregoing patents, a desirable objective in the formation of prismatic lenses is the reduction of material necessary for formation of said lenses due to a primary cost in the manufacture thereof being the amount of material necessary to form said lenses. The invention provides prismatic lighting panels or prismatic lenses capable of a highly desirable level of light control with a desirable reduction of lamp image relative to lens structures of the prior art, the present lens structures further being capable of manufacture from reduced quantities of acrylic or other suitable materials used in the formation of prismatic lens structures. Prismatic lens structures produced according to the invention therefore retain desirable operational characteristics and can be produced at relatively low costs.